Pyrrolo[2,1-c][1,4]benzodiazepines (PBDs), a group of potent naturally occurring antitumour antibiotics from various Streptomyces species, are of considerable interest because of their ability to recognize and subsequently form covalent bonds to specific base sequence of double strand DNA (Dervan, P. B. Science 1989, 232, 464; Hurley, L. H. J. Med. Chem. 1989, 32, 2027; Thurston, D. E.; Thompson, A. S. Chem. Br. 1990, 26, 767). Well-known members of this group include anthramycin, DC-81, sibiromycin, tomamycin, chicamycin and neothramycin of A and B (Hurley, L. H. J. Antibiot. 1977, 30, 349; Schimizu, K.; Kawamoto, I.; Tomita, F.; Morimoto, M.; FuJimoto, K. J. Antibiot. 1982, 35, 992.; Lown, J. W.; Joshua, A. V. Biochem. Pharmacol. 1979, 28, 2017; Thurston, D. E.; Bose, D. S. Chem. Rev. 1994, 94, 433.; Molina, P.; Diaz, I.; Tarraga, A. Tetrahedron 1995, 51, 5617.; Kamal, A.; Rao, N. V. Chem. Commun. 1996, 385; Kamal, A.; Reddy, B. S. P.; Reddy, B. S, N. Tetrahedron Lett 1996, 37, 6803). The cytotoxicity and antitumour activity of these agents are attributed to their property of sequence selective covalent binding to the N2 of guanine in the minor groove of duplex DNA via an acid-labile aminal bond to the electrophilic imine at the N10-C11 position (Kunimoto, S.; Masuda, T.; Kanbayashi, N.; Hamada, M.; Naganawa, H.; Miyamoto, M.; Takeuchi, T.; Unezawa, H. J. Antibiot., 1980, 33, 665.; Kohn, K. W. and Speous, C. L. J. Mol. Biol., 1970, 51, 551.; Hurley, L. H.; Gairpla, C. and ZmiJewski, M. Biochem. Biophys. Acta., 1977, 475, 521.; Kaplan, D. J. and Hurley, L. H. Biochemistry, 1981, 20, 7572). The molecules have a right-handed twist, which allows them to follow the curvature of the minor groove of B-form double-stranded DNA spanning three base pairs. A recent development has been the linking of two PBD units through their C-8 positions to give bisfunctional-alkylating agents capable of cross-linking DNA (Thurston, D. E.; Bose, D. S.; Thomson, A. S.; Howard, P. W.; Leoni, A.; Croker, S. j.; Jenkins, T. C.; Neidle, S, and Hurley, L. H. J. Org. Chem. 1996, 61, 8141).

Recently, PBD dimers have been developed that comprise of two C2-exo-methylene substituted DC-81 subunits tethered through their C-8 position via an inert propanedioxy linker (Gregson, S. J.; Howard, P. W.; Hartely, J. A.; Brooks, N. A.; Adams, L. J.; Jenkins, T. C.; Kelland, L. R. and Thurston, D. E. J. Med. Chem. 2001, 44, 737). A non-cross-linking mixed imine-amide PBD dimers have been synthesized that have significant DNA binding ability and potent antitumour activity (Kamal, A.; Ramesh, G. Laxman, N.; Ramulu, P.; Srinivas, O.; Neelima, K.; Kondapi, A. K.; Srinu, V. B.; Nagarajaram, H. M. J. Med. Chem. 2002, 45, 4679). However, the clinical efficacy for these antibiotics is hindered by several limitations, such as poor water solubility, cardio toxicity, development of drug resistance and metabolic inactivation. Due to the excellent activity of these molecules, there is needed to develop novel derivatives which are devoid of above limitations.
Several A-ring-modified analogues of the DNA-binding antitumor agent DC-81 have been synthesized in order to study structure-reactivity/cytotoxicity relationships. A-ring replaced with pyridine, diazine, pyrimidine, pyrazole and indole rings to give the novel pyrrolo[2,1-c][1,4]pyridodiazepine, pyrrolo[2,1-c][1,4]pyrazolodiazepine, pyrrolo[2,1-c][1,4]diazinodiazepine, pyrrolo[2,1-c][1,4]pyrimidinodiazepine and pyrrolo[2,1-c][1,4]indolodiazepine systems, respectively. Replacement of a benzene ring with naphthalene ring dramatically increases both the DNA binding affinity and cytotoxicity of pyrrolo naphtho diazepine monomers (PND), as can be seen from the ΔTm and IC50 values. On the basis of the increased cytotoxicity of the compound, it would appear that the best position for joining fused ring systems of a PBD is furthest from the bridge carbons.